1. Field of the Invention
This invention relates to a method for simultaneous addition of aluminum and calcium and alloys thereof to molten lead for the production of lead-calcium alloys.
2. Prior Art
Lead acid batteries historically have been produced with lead-antimony alloy grids. These lead-antimony batteries generate hydrogen gas through a chemical reaction with water in the electrolyte. This necessitates venting the battery and periodic replenishment of the water.
The introduction of lead-calcium alloy grids was a major development since these alloys greatly reduce the amount of hydrogen gas generated by the electrochemical reaction. As little as 0.1% calcium in the lead alloy is sufficient to reduce gassing to a level where the battery can virtually be sealed and no water additions are required.
The addition of calcium metal to lead is rather straightforward and can be accomplished by several methods including the simple addition of pure calcium metal or calcium alloys to the surface of a lead bath. Stirring the lead promotes better dissolution and minimizes opportunities for calcium oxidation as it floats on the surface of the lead melt.
U.S. Pat. No. 3,741,754, which issued Jun. 26, 1973 to C. M. Mainland, describes a process for adding granulated calcium metal to lead under an inert protective gas covering thereby minimizing oxidation of calcium during the addition. This process has the advantage of enabling calcium to be added to lead with high recovery and minimal fuming, emissions, flaring and losses to oxidation.
Alternatively, calcium may be introduced by adding calcium carbide as in U.S. Pat. No. 1,941,534 to Betterton, which issued Jan. 2, 1934.
The metallurgical difficulty with producing lead-calcium alloys is not in general related to the addition of calcium to molten lead since, as discussed above, there are several alternative alloying methods. The major problems relate to the fade of calcium once it has been dissolved in the liquid lead pool. Lead-calcium alloys are very prone to the loss of calcium due to oxidation at the melt surface where reactive calcium dissolved in the molten lead comes in contact with oxygen in atmospheric air.
A solution to the problems associated with calcium fade is given by the addition of minor amounts of aluminum (0.005% to 0.05%) to the lead-calcium alloy. Aluminum forms a tenacious oxide layer on the surface of the lead alloy melt thereby minimizing the oxidation of calcium.
Unlike calcium, however, the addition of aluminum to molten lead is quite difficult, being hindered by two factors:
a) a tenacious thin oxide film on the surface of solid aluminum metal which retards its dissolution rate in lead, and PA1 b) aluminum's extremely low solubility in molten lead. At typical lead processing temperatures (400.degree.-600.degree. C.), the maximum solubility of aluminum in molten lead is only marginally higher than specifications for calcium-aluminum-lead alloys (up to 0.05% Al).
To add pure solid aluminum metal to lead effectively, the molten lead temperature has to be raised above aluminum's melting point (660.degree. C.) which is considerably higher than the normal range for lead processing. Due to its reactive nature, calcium metal is usually added at about 420.degree. C. Hence there is an incompatibility between the melt temperatures at which solid aluminum and calcium metals can be added. Alternatively, premelted liquid aluminum can be poured and stirred into the lead bath; however, this requires a second furnace to melt the aluminum.
All of these higher temperature addition methods suffer from excessive oxidation, flaring and fuming leading to health and environmental concerns due to the presence of lead oxide fumes in the atmosphere. Because of the high melt temperatures, calcium recoveries are generally low, averaging below 85%, with aluminum recoveries ranging between 45% to 65%.
Other more environmentally acceptable methods have been developed, the most common utilizing the simultaneous addition of calcium and aluminum in the form of a eutectic alloy. An example of such a process is described in U.S. Pat. No. 4,439,398 to Prengaman issued Mar. 27, 1984. A suitable low melting point (545.degree. C.) eutectic alloy forms at 73% Ca/27% Al.
The eutectic alloy allows for simultaneous addition of calcium and aluminum in a ratio of approximately 3 Ca:1 Al at molten lead temperatures of about 570.degree. C. Typically, reagent recoveries with this alloy are higher than for the pure calcium and aluminum metal addition practices outlined above; calcium and aluminum recoveries of 90% and 70% respectively can be expected with the alloy. The major difficulty arising with the use of this alloy is, however, that the addition ratio is 3 Ca:1 Al which often does not correspond to the proportion of calcium and aluminum required by the lead alloy specifications. In many cases, the desired Ca:Al ratio in the final lead alloy can be as high as 10:1. Achieving this addition ratio is difficult with an alloy of Ca and Al since the melting points of these alloys increase sharply on either side of the 73% Ca, 27% Al eutectic. These elevated melting points make off-eutectic alloys difficult to produce and difficult to use since the lead bath temperature must be increased accordingly. The disadvantages of higher lead temperatures include higher energy costs, longer processing times, shorter kettle life, higher calcium fade and increased lead oxide fume emissions.
Other technologies have also been developed to add aluminum and calcium to molten lead. U.S. Pat. No. 4,808,376, issued Feb. 28, 1989 to Worcester et al., adds calcium and aluminum powders by injecting a mixture carried by a stream of inert gas through a hollow lance which is submerged in a bath of molten lead. Because of the large density difference between the calcium and aluminum powders and liquid lead, the lance has to be equipped with a perforated canister at its tip. This canister acts to capture and hold the rising calcium and aluminum powders as they exit the lance. With this technique, calcium and aluminum recoveries are typically 90% and 55% respectively. This process still suffers from low aluminum recoveries and requires complicated and costly submerged injection equipment including the use of lances which are prone to plugging problems.
U.S. Pat. No. 4,627,961, issued Dec. 9, 1986 to Dudek, calls for the simultaneous addition of calcium and aluminum by compressing a mechanical mixture of the respective metal granules into a briquette. The briquettes are then added into a vortex created by stirring molten lead at temperatures between 550.degree. and 600.degree. C. The major difficulty with this method is unpredictability in the amount of calcium and aluminum recovered in the melt. Trials with calcium/aluminum briquettes, described in column 1 of aforesaid U.S. Pat. No. 4,808,376, show highly variable results with calcium recoveries varying by up to 10% and aluminum recoveries varying by over 34%.
U.S. Pat. No. 4,699,764, issued Oct. 13, 1987 to Tobias et al., describes a complicated system where lead is melted in a separate holding furnace which is connected to a dissolution furnace by heated feed and return pipes. A pump is used to move the molten lead through the heated feed pipe in which the lead temperature is increased to the desired alloying temperature. The heated lead then passes through the dissolution furnace where it alloys with the addition reagents such as aluminum. The alloyed lead is then returned via the second pipe to the main lead holding furnace. The molten lead is continually circulated until the desired alloy composition is attained. This process is not only complicated but uses pumps and heated pipes which are maintenance intensive items.
In summary, while the addition of calcium metal to lead is relatively straightforward with recoveries of 90% or better expected, the addition of aluminum metal to molten lead is quite difficult. All processes which add pure aluminum metal at melt temperatures of 660.degree. C. or less (that is below the melting point of aluminum) suffer from low and highly variable aluminum recoveries. Many require complicated, costly and maintenance intensive equipment. At lead melt temperatures above aluminum's melting point, aluminum recoveries improve but the excessive heat leads to higher energy costs, longer processing times, shorter kettle life, higher calcium fade and excessive fuming of lead oxide causing serious environmental concerns.